Grey water is the waste water from showers, baths, spas, hand-basins, laundry tubs, washing machines, dishwashers and kitchen sinks. It doesn't include water from toilets – that's called black water.

With water restrictions in many areas, using grey water on your garden could help save hundreds of litres of water a day, keeping your wallet happy and the planet happier still.

Washing machines account for almost a quarter of household wastewater or, depending on your machine, about 60–180 litres per wash. So, in effect, washing six times a week could send more than 1000L down the drain in one week alone. Add to that your dishwashing, shower and bath water, and you're soon up to 4000L a week for the average family of four.

How do you use grey water?

Grey water can be stored and used on the garden (or even in toilets or washing machines), or else it can be diverted to the garden with a plumbed-in diverter (it's basically a switch, so if it's raining it goes into the sewer instead). Conditions may apply in the area where you live so contact your local council for advice. DIY options include attaching an extra-long flexible hose from the washing machine to the garden, or using a bucket.

Dangerous grey water liaisons

  • Don't leave buckets lying around if you have small children – they're a drowning hazard.
  • Don't store untreated grey water for more than 24 hours; if you can't use it, don't keep it.
  • If it's untreated, limit your use to water from the shower, bath and washing machine (preferably only the rinse water). Kitchen water contains fats and solids that might damage your soil and plants.
  • If someone in your family is sick with gastro, flu or another contagious disease, stop using the grey water till they're better.
  • Don't water herbs, vegetables, or pot plants without access to other kinds of moisture.
  • Keep the grey water underground, or under mulch – this helps prevent evaporation and keeps it away from kids and pets.

Washing machine water

In our laundry detergent review we tested the washing water for chemicals that could harm your garden. We found the components most likely to cause problems are phosphorus, salinity, sodium and pH to varying degrees.

Phosphorus

Small amounts of phosphorus can be useful for plants, and it's a major component of fertiliser. When it gets into our waterways, however, it can cause excessive algae growth, leading to toxic algal blooms. The effect on your soil is varied depending on your soil type.

  • Clay soils can deal with more phosphorus because the phosphorus binds to clay minerals and doesn't leach away.
  • On sandy soils, excess phosphorus can leach into groundwater.
  • Australian soils are typically low in phosphorus, and some native species can't tolerate high levels.

Salts

All laundry detergents contain salts, typically sodium salts such as sodium nitrate, sodium sulphate, sodium phosphate and sodium silicate. All laundry detergents are highly saline, and frequent long-term use would likely harm your garden, unless it was spread over a large area.

Sodium in the salts is particularly detrimental not only to plants, but soils. It affects the soil's permeability and causes a loss of structural stability.

Alkalinity

Laundry detergents are highly alkaline (that is, detergents that have a high pH). A pH that's higher than 10 helps dissolve organic dirt, such as grease, oils and food scraps. Most biological systems prefer a pH between 6 and 9, and grey water with a high pH is likely to harm many plants and soil organisms.

Our review considers the total load of problem chemicals that will accumulate in your garden over time, not just their concentration when you first put them on. The larger the irrigation area (minimum recommended area is 150–200 m2), the more you'll spread the chemical load.

Potential impacts are very much dose-dependent – try reducing the amount of detergent you use, providing the reduced amount will still get your clothes clean enough.

If you want to install a grey water pre-treatment system, do all of the above, plus:

  • consult your sewage removal authority if you intend to redirect all or a major part of your used water
  • consult a licensed plumber for advice on the best system for your needs
  • inform your water supply authority of any changes to your plumbing.